Both V(D)J coding ends but neither signal end can recombine at the bcl-2 major breakpoint region, and the rejoining is ligase IV dependent

The t(14;18) chromosomal translocation is the most common translocation in human cancer, and it occurs in all follicular lymphomas. The 150-bp bcl-2 major breakpoint region (Mbr) on chromosome 18 is a fragile site, because it adopts a non-B DNA conformation that can be cleaved by the RAG complex. The non-B DNA structure and the chromosomal translocation can be recapitulated on intracellular human minichromosomes where immunoglobulin 12- and 23-signals are positioned downstream of the bcl-2 Mbr. Here we show that either of the two coding ends in these V(D)J recombination reactions can recombine with either of the two broken ends of the bcl-2 Mbr but that neither signal end can recombine with the Mbr. Moreover, we show that the rejoining is fully dependent on DNA ligase IV, indicating that the rejoining phase relies on the nonhomologous DNA end-joining pathway. These results permit us to formulate a complete model for the order and types of cleavage and rejoining events in the t(14;18) translocation.

Oh what a tangled web it weaves: BRCA1 and DNA decatenation

BRCA1 has significant roles in DNA repair and cell cycle checkpoint control, and is important in the maintenance of genomic stability. Defects in these pathways likely underpin the cancer susceptibility of BRCA1 mutation carriers. Now, a new function for BRCA1 in DNA decatenation--removing the tangles introduced into chromosomes as a consequence of DNA replication--is suggested in a new paper by Lou et al. (2005) in Nature Structural and Molecular Biology. Ineffective DNA decatenation may lead to chromosome breakage and inappropriate repair, adding to the roll call of defects in BRCA1 mutant cells.

Genome-wide characterization of tetrahymena thermophila chromosome breakage sites. I. Cloning and identification of functional sites

The chromosomes of the macronuclear (expressed) genome of Tetrahymena thermophila are generated by developmental fragmentation of the five micronuclear (germline) chromosomes. This fragmentation is site specific and directed by a conserved 15-bp chromosome breakage sequence (Cbs element). This article reports the construction of a library enriched for chromosome breakage junctions and the development of a successful scheme for the genome-wide isolation and characterization of functional Cbs junctions. Twenty-three new Cbs junctions were characterized and each was assigned to a specific micronuclear chromosome or chromosome arm. Two distinct previously unreported variant chromosome breakage sequences were found, each in two or more functional Cbs elements. Analysis of natural Cbs junctions confirmed that microheterogeneity in the macronuclear telomere addition site is associated with chromosome fragmentation. The physical and genetic characterization of these functional chromosome breakage junctions is reported in the accompanying article in this issue. The whole-genome shotgun sequencing and auto-assembly phase of the Tetrahymena Genome Initiative has recently been completed at The Institute for Genome Research (TIGR). By providing unique sequence from the natural ends of macronuclear chromosomes, Cbs junctions characterized in the work reported here will serve as useful sequence tags for relating macro- and micronuclear genetic, physical, and sequence maps.

Disruption of a brain transcription factor, NPAS3, is associated with schizophrenia and learning disability

A mother and daughter diagnosed with schizophrenia and schizophrenia co-morbid with mild learning disability, respectively, possess a balanced reciprocal translocation t(9,14)(q34.2;q13). Fluorescence in situ hybridization (FISH) with YAC, BAC, and cosmid probes indicate that the chromosome 14q13 breakpoint disrupts a large gene, NPAS3, encoding a CNS expressed transcription factor of the basic helix-loop-helix PAS (bHLH-PAS) gene family. By analogy with other members of the bHLH-PAS family, the putative truncated protein generated from the disrupted gene locus may have a dominant negative effect. The 14q13 region was previously identified by a linkage study of an inherited neurodegenerative condition, idiopathic basal ganglia calcification (IBGC or Fahr syndrome, OMIM:213600/606656), which is often co-morbid with psychosis. Sequencing of the gene in a third patient diagnosed with IBGC, schizophrenia, and mild learning disability did not reveal functional mutations.

Factors affecting double-strand break-induced homologous recombination in mammalian cells

Double-strand break (DSB)-induced homologous recombination (HR) of direct repeats is a powerful means to achieve gene excision, a critical step in genome engineering. In this report we have used an extrachromosomal reporter system to monitor the impact of different parameters on meganuclease-induced HR in CHO-K1 cells. We found that repeat homology length is critical. Virtually no HR could be detected with a 15-bp duplication, while, with repeats larger than 400 bp, recombination efficiency became less dependent on homology length. The presence of an intervening sequence between the duplications dramatically impairs HR, independent of the cleavage position; by 3 kb of insertion, HR is virtually undetectable. Efficient HR can be restored by positioning cleavage sites at both ends of the intervening sequence, allowing a constant level of excision with up to 10 kb of intervening sequences. Using similar constructs, 2.8-kb inserts could be efficiently removed from several chromosomal loci, illustrating the wide potential of this technology. These results fit current models of direct repeat recombination and identify DSB-induced HR as a powerful tool for gene excision.

Non-random structural chromosomal changes in primary gastric cancer

Gastric cancer is of major importance world-wide representing the second most common cause of cancer related death in the world. Data concerning the chromosomal changes in gastric cancer are limited and no specific change has been identified to date. We have studied cytogenetically 15 cases of primary gastric cancer by direct culture of tumors cells and G-banding technique. We focused on structural aberrations observed in order to identify non-random aberrations presenting common chromosomal breakpoints. Chromosomes most commonly involved were according to frequency 1,11,14,7,17,6,8 and 13. Chromosome 11 was involved as add(11)(p15), while the pericentromeric area of chromosome 14 was constantly participated in aberrations. Isochromosomes i(1q), i(8q), i(13q), i(14q) and i(17q) were constantly found. Furthermore translocations t(1;7), t(7;14), t(6;17) and t(5;14) were identified. Conventional cytogenetics continues to be valuable in cancer study detecting genomic areas potentially candidate for the isolation of genes related to carcinogenesis.

In vivo DNA double-strand breaks enhance gene targeting in cultured silkworm cells

Alteration of genomic information through homologous recombination (HR) is a powerful tool for reverse genetics in bacteria, yeast, and mice. The low frequency of HR is, however, a major obstacle to achieve efficient gene targeting. In this study, we have developed an assay system for investigating the frequency of gene targeting in cultured silkworm cells using a firefly luciferase gene as a reporter. The introduction of a DNA double-strand break (DSB) either in the chromosomal target locus or in the targeting construct drastically increased the frequency of gene targeting. Interestingly, the inhibition of poly(ADP-ribose) polymerase (PARP), a protein known to play an important role in overall suppression of the HR pathway, stimulated the targeting efficiency, whereas the overexpression of two silkworm RecA homologs, BmRad51 and BmDmc1, had no effect. The presently devised assay system may serve as a useful tool to improve the gene targeting efficiency in the silkworm (Bombyx mori).

Breakpoints around the HOXD cluster result in various limb malformations

BACKGROUND

Characterisation of disease associated balanced chromosome rearrangements is a promising starting point in the search for candidate genes and regulatory elements.

METHODS

We have identified and investigated three patients with limb abnormalities and breakpoints involving chromosome 2q31. Patient 1 with severe brachydactyly and syndactyly, mental retardation, hypoplasia of the cerebellum, scoliosis, and ectopic anus, carries a balanced t(2;10)(q31.1;q26.3) translocation. Patient 2, with translocation t(2;10)(q31.1;q23.33), has aplasia of the ulna, shortening of the radius, finger anomalies, and scoliosis. Patient 3 carries a pericentric inversion of chromosome 2, inv(2)(p15q31). Her phenotype is characterised by bilateral aplasia of the fibula and the radius, bilateral hypoplasia of the ulna, unossified carpal bones, and hypoplasia and dislocation of both tibiae.

RESULTS

By fluorescence in situ hybridisation, we have mapped the breakpoints to intervals of approximately 170 kb or less. None of the three 2q31 breakpoints, which all mapped close to the HOXD cluster, disrupted any known genes.

CONCLUSIONS

Hoxd gene expression in the mouse is regulated by cis-acting DNA elements acting over distances of several hundred kilobases. Moreover, Hoxd genes play an established role in bone development. It is therefore very likely that the three rearrangements disturb normal HOXD gene regulation by position effects.

Comparative cytogenetics of human chromosome 3q21.3 reveals a hot spot for ectopic recombination in hominoid evolution

Fluorescence in situ hybridization mapping of fully integrated human BAC clones to primate chromosomes, combined with precise breakpoint localization by PCR analysis of flow-sorted chromosomes, was used to analyze the evolutionary rearrangements of the human 3q21.3-syntenic region in orangutan, siamang gibbon, and silvered-leaf monkey. Three independent evolutionary breakpoints were localized within a 230-kb segment contained in BACs RP11-93K22 and RP11-77P16. Approximately 200 kb of the human 3q21.3 sequence was not present on the homologous orangutan, siamang, and Old World monkey chromosomes, suggesting a genomic DNA insertion into the breakpoint region in the lineage leading to humans and African great apes. The breakpoints in the orangutan and siamang genomes were narrowed down to 12- and 20-kb DNA segments, respectively, which are enriched with endogenous retrovirus long terminal repeats and other repetitive elements. The inserted DNA segment represents part of an ancestral duplication. Paralogous sequence blocks were found at human 3q21, approximately 4 Mb proximal to the evolutionary breakpoint cluster region; at human 3p12.3, which contains an independent orangutan-specific breakpoint; and at the subtelomeric and pericentromeric regions of multiple human and orangutan chromosomes. The evolutionary breakpoint regions between human chromosome 3 and orangutan 2 as well their paralogous segments in the human genome coincide with breaks of chromosomal synteny in the mouse, rat, and/or chicken genomes. Collectively our data reveal reuse of the same short recombinogenic DNA segments in primate and vertebrate evolution, supporting a nonrandom breakage model of genome evolution.

Telomere fusion to chromosome breaks reduces oncogenic translocations and tumour formation

Telomeres protect chromosome ends from fusion, degradation and recombination. Loss of telomere function has opposite effects on tumorigenesis: apoptosis, which inhibits tumour growth, and genomic instability, which accelerates tumour formation. Here we describe a new mechanism by which short telomeres inhibit tumorigenesis through interference with oncogenic translocations. In mice that are null for both ataxia-telangiectasia-mutated (Atm) and telomerase RNA (mTR), the first generation (G1) Atm-/- mTR-/- mice have a lower rate of tumour formation than Atm-/- mTR+/+ mice. These Atm-/- mTR-/- G1 tumours show no increase in either apoptosis or overall genomic instability. Strikingly, the tumours show a high fraction of translocations containing telomere signals at the translocation junctions. Translocations of the T-cell receptors on chromosome 14, which initiate tumorigenesis, were interrupted by fusion with telomeres. Telomere repeats were also detected at the translocation junctions in pre-malignant thymocytes. We propose that telomere fusion to DNA double-strand breaks competes with the generation of oncogenic translocations and thus reduces tumour formation.

Genomic structure and paralogous regions of the inversion breakpoint occurring between human chromosome 3p12.3 and orangutan chromosome 2

Intrachromosomal duplications play a significant role in human genome pathology and evolution. To better understand the molecular basis of evolutionary chromosome rearrangements, we performed molecular cytogenetic and sequence analyses of the breakpoint region that distinguishes human chromosome 3p12.3 and orangutan chromosome 2. FISH with region-specific BAC clones demonstrated that the breakpoint-flanking sequences are duplicated intrachromosomally on orangutan 2 and human 3q21 as well as at many pericentromeric and subtelomeric sites throughout the genomes. Breakage and rearrangement of the human 3p12.3-homologous region in the orangutan lineage were associated with a partial loss of duplicated sequences in the breakpoint region. Consistent with our FISH mapping results, computational analysis of the human chromosome 3 genomic sequence revealed three 3p12.3-paralogous sequence blocks on human chromosome 3q21 and smaller blocks on the short arm end 3p26-->p25. This is consistent with the view that sequences from an ancestral site at 3q21 were duplicated at 3p12.3 in a common ancestor of orangutan and humans. Our results show that evolutionary chromosome rearrangements are associated with microduplications and microdeletions, contributing to the DNA differences between closely related species.

Evolutionary breakpoint analysis on Y chromosomes of higher primates provides insight into human Y evolution

Comparative FISH mapping of PAC clones covering almost 3 Mb of the human AZFa region in Yq11.21 to metaphases of human and great apes unravels breakpoints that were involved in species-specific Y chromosome evolution. An astonishing clustering of evolutionary breakpoints was detected in the very proximal region on the long arm of the human Y chromosome in Yq11.21. These breakpoints were involved in deletions, one specific for the human and another for the orang-utan Y chromosome, in a duplicative translocation/transposition specific for bonobo and chimpanzee Y chromosomes and in a pericentric inversion specific for the gorilla Y chromosome. In addition, our comparative results allow the deduction of a model for the human Y chromosome evolution.

Evolutionary breakpoints are co-localized with fragile sites and intrachromosomal telomeric sequences in primates

The concentration of evolutionary breakpoints in primate karyotypes in some particular regions or chromosome bands suggests that these chromosome regions are more prone to breakage. This is the first extensive comparative study which investigates a possible relationship of two genetic markers (intrachromosomal telomeric sequences [TTAGGG]n, [ITSs] and fragile sites [FSs]), which are implicated in the evolutionary process as well as in chromosome rearrangements. For this purpose, we have analyzed: (a) the cytogenetic expression of aphidicolin-induced FSs in Cebus apella and Cebus nigrivittatus (F. Cebidae, Platyrrhini) and Mandrillus sphinx (F. Cercopithecidae, Catarrhini), and (b) the intrachromosomal position of telomeric-like sequences by FISH with a synthetic (TTAGGG)n probe in C. apella chromosomes. The multinomial FSM statistical model allowed us to determinate 53 FSs in C. apella, 16 FSs in C. nigrivittatus and 50 FSs in M. sphinx. As expected, all telomeres hybridized with the probe, and 55 intrachromosomal loci were also detected in the Cebus apella karyotype. The chi(2) test indicates that the coincidence of the location of Cebus and Mandrillus FSs with the location of human FSs is significant (P < 0.005). Based on a comparative cytogenetic study among different primate species we have identified (or described) the chromosome bands in the karyotypes of Papionini and Cebus species implicated in evolutionary reorganizations. More than 80% of these evolutionary breakpoints are located in chromosome bands that express FSs and/or contain ITSs.

Nucleotide sequence comparison of a chromosome rearrangement on human chromosome 12 and the corresponding ape chromosomes

Chromosome rearrangement has been considered to be important in the evolutionary process. Here, we demonstrate the evolutionary relationship of the rearranged human chromosome 12 and the corresponding chromosome XII in apes (chimpanzee, bonobo, gorilla, orangutan, and gibbon) by examining PCR products derived from the breakpoints of inversions and by conducting shotgun sequencing of a gorilla fosmid clone containing the breakpoint and a "duplicated segment" (duplicon). We confirmed that a pair of 23-kb duplicons flank the breakpoints of inversions on the long and short arms of chimpanzee chromosome XII. Although only the 23-kb duplicon on the long arm of chimpanzee chromosome XII and its telomeric flanking sequence are found to be conserved among the hominoids (human, great apes, and gibbons), the duplicon on the short arm of chimpanzee chromosome XII is suggested to be the result of a duplication from that on the long arm. Furthermore, the shotgun sequencing of a gorilla fosmid indicated that the breakpoint on the long arm of the gorilla is located at a different position 1.9 kb from that of chimpanzee. The region is flanked by a sequence homologous to that of human chromosome 6q22. Our findings and sequence analysis suggest a close relationship between segmental duplication and chromosome rearrangement (or breakpoint of inversion) in Hominoidea. The role of the chromosome rearrangement in speciation is also discussed based on our new results.

Disruption of TCBA1 associated with a de novo t(1;6)(q32.2;q22.3) presenting in a child with developmental delay and recurrent infections

A boy with developmental delay, particularly of speech, a distinct face, antineutrophil cytoplasmic antibodies, and recurrent infections was found to have an apparently balanced de novo t(1;6)(q32.3;q22.3) translocation. Fluorescent in situ hybridisation with BAC/PAC clones and long range polymerase chain reaction products assessed in the human genome sequence localised the chromosome 1 breakpoint to a 9.8 kb segment within a hypothetical gene, LOC388735, and the chromosome 6 breakpoint to a 12.8 kb segment in intron 4 of the T-cell lymphoma breakpoint-associated target 1 (TCBA1) gene. Disruption and/or formation of TCBA1 fusion genes in T cell lymphoma and leukaemia cell lines suggests a role for this gene in tumorigenesis. The isolated mouse Tcba1 gene shows 91% amino acid sequence similarity with human TCBA1. It is expressed in fetal and adult brain and with lower levels in liver and testis. The human gene has been reported to be expressed exclusively in brain and thymus. Reduced TCBA1 expression in brain and thymus may explain at least some of the symptoms in this patient. It is concluded that germline alterations of the TCBA1 gene are associated with developmental delay and typical physical features.

Chromosomal radiosensitivity in BRCA1 and BRCA2 mutation carriers

PURPOSE

The chromosomal radiosensitivity of a selected group of familial breast cancer patients carrying a mutation in BRCA1 (n=11) or BRCA2 (n=9) and a group of healthy mutation carriers (n=12) was investigated and compared to a reference group of breast cancer patients without a BRCA1/2 mutation (n=78) and a group of healthy women carrying no mutation (n=58).

MATERIALS AND METHODS

The chromosomal radiosensitivity was assessed with the G2 and the G0-micronucleus (MN)-assay on fresh blood samples and on Epstein-Barr virus (EBV)-transformed lymphoblastoid cell lines. For the MN-assay, lymphocytes were exposed in vitro to 3.5 Gy and 2 Gy 60Co gamma-rays at a high dose rate (HDR) or low dose rate (LDR). 70-h post-irradiation cultures were arrested and micronuclei were scored in 1000 binucleate cells. For the G2-assay lymphocytes were irradiated in vitro with a dose of 0.4 Gy 60Co gamma-rays after 71h incubation. Cultures were arrested 90 min after irradiation and chromatid breaks were scored in 50 metaphases.

RESULTS

The group of breast cancer patients with a BRCA1 or 2 mutation was on average more radiosensitive than the control group, but not different from breast cancer patients without a BRCA mutation. The radiation response of healthy BRCA1/2 carriers was not significantly different from the control group and also not different from relatives without a BRCA mutation. Comparing the radiation response in EBV cell lines derived from breast cancer patients with or without a BRCA1 mutation revealed no significant difference.

CONCLUSIONS

Our results reveal that chromosomal radiosensitivity observed in breast cancer patients heterozygous for BRCA1 or 2 mutations, could not be demonstrated in healthy BRCA1/2 mutation carriers. This suggests that mutations in BRCA1 or 2 genes are not playing a main role in chromosomal radiosensitivity, this although BRCA1 and 2 are both involved in DNA repair/signalling processes.

Molecular breakpoint cloning and gene expression studies of a novel translocation t(4;15)(q27;q11.2) associated with Prader-Willi syndrome

BACKGROUND

Prader-Willi syndrome (MIM #176270; PWS) is caused by lack of the paternally-derived copies, or their expression, of multiple genes in a 4 Mb region on chromosome 15q11.2. Known mechanisms include large deletions, maternal uniparental disomy or mutations involving the imprinting center. De novo balanced reciprocal translocations in 5 reported individuals had breakpoints clustering in SNRPN intron 2 or exon 20/intron 20. To further dissect the PWS phenotype and define the minimal critical region for PWS features, we have studied a 22 year old male with a milder PWS phenotype and a de novo translocation t(4;15)(q27;q11.2).

METHODS

We used metaphase FISH to narrow the breakpoint region and molecular analyses to map the breakpoints on both chromosomes at the nucleotide level. The expression of genes on chromosome 15 on both sides of the breakpoint was determined by RT-PCR analyses.

RESULTS

Pertinent clinical features include neonatal hypotonia with feeding difficulties, hypogonadism, short stature, late-onset obesity, learning difficulties, abnormal social behavior and marked tolerance to pain, as well as sticky saliva and narcolepsy. Relative macrocephaly and facial features are not typical for PWS. The translocation breakpoints were identified within SNRPN intron 17 and intron 10 of a spliced non-coding transcript in band 4q27. LINE and SINE sequences at the exchange points may have contributed to the translocation event. By RT-PCR of lymphoblasts and fibroblasts, we find that upstream SNURF/SNRPN exons and snoRNAs HBII-437 and HBII-13 are expressed, but the downstream snoRNAs PWCR1/HBII-85 and HBII-438A/B snoRNAs are not.

CONCLUSION

As part of the PWCR1/HBII-85 snoRNA cluster is highly conserved between human and mice, while no copy of HBII-438 has been found in mouse, we conclude that PWCR1/HBII-85 snoRNAs is likely to play a major role in the PWS- phenotype.

Gene conversion and evolution of Xq28 duplicons involved in recurring inversions causing severe hemophilia A

Inversions breaking the 1041 bp int1h-1 or the 9.5-kb int22h-1 sequence of the F8 gene cause hemophilia A in 1/30,000 males. These inversions are due to homologous recombination between the above sequences and their inverted copies on the same DNA molecule, respectively, int1h-2 and int22h-2 or int22h-3. We find that (1) int1h and int22h duplicated more than 25 million years ago; (2) the identity of the copies (>99%) of these sequences in humans and other primates is due to gene conversion; (3) gene conversion is most frequent in the internal regions of int22h; (4) breakpoints of int22h-related inversions also tend to involve the internal regions of int22h; (5) sequence variations in a sample of human X chromosomes defined eight haplotypes of int22h-1 and 27 of int22h-2 plus int22h-3; (6) the latter two sequences, which lie, respectively, 500 and 600 kb telomeric to int22h-1 are five-fold more identical when in cis than when in trans, thus suggesting that gene conversion may be predominantly intrachromosomal; (7) int1h, int22h, and flanking sequences evolved at a rate of about 0.1% substitutions per million years during the divergence between humans and other primates, except for int1h during the human-chimpanzee divergence, when its rate of evolution was significantly lower. This is reminiscent of the slower evolution of palindrome arms in the male specific regions of the Y chromosome and we propose, as an explanation, that intrachromosomal gene conversion and cosegregation of the duplicated regions favors retention of the ancestral sequence and thus reduces the evolution rate.

[Chromosomal rearrangements breakpoints clusterization: is the clonal selection involved]

Certain types of cancer are often correlate with certain chromosomal rearrangements. The chimaeric genes are formed as a result of this rearrangements, and the chimaeric proteins are the products of their expression. The breakpoints of such translocation are often clustered in the genome. Moreover, such breakpoint clusters often contain specific genomic elements like topoisomerase II consensus sites, nuclear matrix attachment regions and DNA sequences, which can make up secondary non-canonical structure. In this review we discuss whether breakpoints may be induced by chromatin structure. Furthermore, we bring up not touched upon literature question about the relation between the breakpoint clusters and the domain organization of corresponding proteins. We also consider possible mechanisms of chromosomal rearrangements.

Increased Rates of Chromosome Breakage in BRCA1 Carriers Are Normalized by Oral Selenium Supplementation

Women who are born with constitutional heterozygous mutations of the BRCA1 gene face greatly increased risks of breast and ovarian cancer. The product of the BRCA1 gene is involved in the repair of double-stranded DNA breaks and it is believed that increased susceptibility to DNA breakage contributes to the cancer phenotype. It is hoped therefore that preventive strategies designed to reduce chromosome damage will also reduce the rate of cancer in these women. To test for increased mutagenicity of cells from BRCA1 carriers, the frequency of chromosome breaks was measured in cultured blood lymphocytes following in vitro exposure to bleomycin in female BRCA1 carriers and was compared with noncarrier relatives. The frequency of chromosome breaks was also measured in BRCA1 carriers following oral selenium supplementation. Carriers of BRCA1 mutations showed significantly greater mean frequencies of induced chromosome breaks per cell than did healthy noncarrier relatives (0.58 versus 0.39; P < 10(-4)). The frequency of chromosome breaks was greatly reduced following 1 to 3 months of oral selenium supplementation (mean, 0.63 breaks per cell versus 0.40; P < 10(-10)). The mean level of chromosome breaks in carriers following supplementation was similar to that of the noncarrier controls (0.40 versus 0.39). Oral selenium is a good candidate for chemoprevention in women who carry a mutation in the BRCA1 gene.

The control of Spo11's interaction with meiotic recombination hotspots

Programmed double-strand breaks (DSBs), which initiate meiotic recombination, arise through the activity of the evolutionary conserved topoisomerase homolog Spo11. Spo11 is believed to catalyze the DNA cleavage reaction in the initial step of DSB formation, while at least a further 11 factors assist in Saccharomyces cerevisiae. Using chromatin-immunoprecipitation (ChIP), we detected the transient, noncovalent association of Spo11 with meiotic hotspots in wild-type cells. The establishment of this association requires Rec102, Rec104, and Rec114, while the timely removal of Spo11 from chromatin depends on several factors, including Mei4 and Ndt80. In addition, at least one further component, namely, Red1, is responsible for locally restricting Spo11's interaction to the core region of the hotspot. In chromosome spreads, we observed meiosis-specific Spo11-Myc foci, independent of DSB formation, from leptotene until pachytene. In both rad50S and com1Delta/sae2Delta mutants, we observed a novel reaction intermediate between Spo11 and hotspots, which leads to the detection of full-length hotspot DNA by ChIP in the absence of artificial cross-linking. Although this DNA does not contain a break, its recovery requires Spo11's catalytic residue Y135. We propose that detection of uncross-linked full-length hotspot DNA is only possible during the reversible stage of the Spo11 cleavage reaction, in which rad50S and com1Delta/sae2Delta mutants transiently arrest.

Structural and numerical chromosome changes in colon cancer develop through telomere-mediated anaphase bridges, not through mitotic multipolarity

Telomere dysfunction has been associated with chromosomal instability in colorectal carcinoma, but the consequences of telomere-dependent instability for chromosome integrity and clonal evolution have been little explored. We show here that abnormally short telomeres lead to a wide spectrum of mitotic disturbances in colorectal cancer cell lines, including anaphase bridging, whole-chromosome lagging, and mitotic multipolarity. These abnormalities were found in both the presence and absence of microsatellite instability. The mean telomere length varied extensively between cells from the same tumor, allowing the establishment of tumor cell subpopulations with highly different frequencies of mitotic disturbances. Anaphase bridging typically resulted in either inter-centromeric chromatin fragmentation or centromere detachment, leading to pericentromeric chromosome rearrangements and loss of whole chromosomes, respectively. There was a strong correlation between anaphase bridges and multipolar mitoses, and the induction of dicentric chromosomes by gamma irradiation and telomerase inhibition led to an elevated frequency of multipolar mitotic spindles, suggesting that multipolarity could result from polyploidization triggered by anaphase bridging. Chromatid segregation in multipolar mitoses was close to random, resulting in frequent nullisomies and nonviable daughter cells. In contrast, there was a high clonogenic survival among cells having gone through anaphase bridging in bipolar mitoses. Bridging of telomere-deficient chromosomes could thus be a major mutational mechanism in colorectal cancer, whereas mitotic multipolarity appears to be a secondary phenomenon that rarely, if ever, contributes to clonal evolution.